In studies of high resolution NMR phenomena one limiting characteristic is the signal-to-noise ratio. The noise has a diverse origin and includes sources other than simple stochastic contributions. It is known that leads of the RF coil may serve as sources of parasitic radiation which may affect a sample under investigation. The present work is predicated upon improvements to RF probe performance incident to incorporating structure having the effect of eliminating, in the sensitive volume of the apparatus, RF signal pick-up originating from coil leads and otherwise shielding the sample from discontinuities of the termination of the coil itself (end effects).
The polarizing field of the NMR apparatus is carefully shimmed to provide for the highest achievable uniformity in the central region of the RF coil where the coil design provides for application of the desired irradiation. The sample is ordinarily of extended dimension along one axis to avoid discontinuity in this sensitive volume, but sample regions removed from the center region of the sensitive volume may experience a significantly different polarizing field and therefore resonate at a different frequency. Steps taken to suppress solvent excitation in the center region of the sensitive volume may be ineffective against spurious excitation from coil leads near the ends of the sample. A broad resonance, shifted from the solvent peak can result, masking relatively weak intensity spectral features in this region.
In prior art, an appreciation of the possibility of parasitic excitation arising from radiation from coil leads was discussed in U.S. Pat. No. 4,851,780. A structure was there described comprising symmetrical excited coil portions with the null point of the coil assembly furnishing a virtual ground for a shield plate disposed between the coil and that portion of the coil leads proximate the coil.
The effect of parasitic RF irradiation from coil leads is especially important in such situations where solvent suppression is required to observe spectral lines close to solvent resonances. With low sample concentration, e.g., 10.sup.-3 the solvent line(s) are inherently about 10.sup.6 times as intense the lines under study. Measures taken for suppression of a solvent resonance assume that such measures (saturation, for example) are ideally achieved. Where the sample has physical extension in regions beyond the sensitive volume and close to coil leads, even very weak radiation of the solvent (in the end portion) of the sensitive volume is sufficient to produce an undesired signal contribution.
The problem is exacerbated in certain composite probe structures. It is not uncommon for two coils to be coaxially disposed with the sample occupying a capillary along the common axis. One example of such structure is the use of quadrature coils. In another common arrangement an inner coil may be provided for .sup.13 C irradiation and the outer coil provided for proton irradiation. Even in the case where such a probe is in use exclusively for proton resonance measurements and the inner coil is nominally inert, the presence of the inner coil leads, close to a portion of the sample (even though far from the center of the sensitive volume) provides a means for re-radiating the proton irradiation on the remote sample portion.
In the present invention shielding of the sample of coil leads is accomplished with shielding comprising a floating conductive guard cylinder and/or an end cover or disk.
Use of a floating guard ring, coaxially disposed inside of a slotted resonator is described in U.S. Ser. No. 07/681,218. This structure was understood to provide capacitive couplings between resonator portions separated by slots, or simply to provide a virtual ground in symmetric geometrical and electrical relationship to an outer ring structure.